In simple words, soil fertility, refers to a soil rich in nutrients. However, in the broader definition, it is determined by the physical and physiochemical properties of the soil. The physical properties include factors such as soil texture, water holding capacity, the depth of the soil profile, drainage etc. Physiochemical properties refer to properties such as the cation exchange capacity (CEC), anion exchange capacity (AEC), pH and the level of available plant nutrients.
Fertile soils also allow minimizing human health risks associated with crop production. For example, it will require less fertilizer and pesticide applications.
A fertile soil will have the following properties:
- It will be rich with organic matter
- Has a high CEC
- pH level is adequate pH (6.0-7.0)
- Well drained.
- Soil has a low salinity level
- Available nutrients are at adequate levels and in proper balance.
- The soil has a good structure and low risk of erosion.
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Some of those properties are difficult to change. However, proper management can improve soil properties, and often high crop productivity can be achieved even in soils that are not naturally fertile. Whereas improper management of a crop growing in a fertile soil might result in poor crop growth and yield, fertile soils can partially compensate for bad practices. On the other hand, mistakes made while growing in an infertile soil might result in damage to the crop.
For example, sandy soils with low organic matter content are not considered to be fertile. However, good yields can still be achieved. Growing high yielding crops in this type of soils requires more intensive application of fertilizers and water and enriching the soil with organic matter.
Another point to remember when talking about soil fertility is that a soil that is considered fertile for one crop may not be fertile for another. This is due to differences in nutrient requirements, pH range, root system depth etc.
The role of microorganisms in soil fertility
Microorganisms and biodiversity play an important role in soil fertility. Microorganisms break down organic matter and convert it into plant-available nutrients, in a process called “mineralization”. A considerable amount of nitrogen can be mineralized and become available for the crop.
Legume crops are capable of fixing atmospheric nitrogen. In this process, which is controlled by soil bacteria that live in symbiosis with legume roots, atmospheric nitrogen is being converted to ammonia and then to nitrite and nitrate.
The availability of soil phosphorous and potassium is assisted by microorganisms. Bacteria species that release organic acids solubilize bound mineral phosphorus and potassium, while other bacteria species mineralize organic phosphorus.
Iron is an essential micronutrient. Microorganisms that release siderophores, organic acids and other exudates help in solubilizing mineral iron and making it available for the crop.
Boron availability is also dependent on soil microorganisms, among other factors. Microorganisms break down soil organic matter and allow the release of boron from organic complexes.
The above are a few examples of the importance of microorganisms to soil fertility.
How to manage and maintain soil fertility
There is a large set of practices to maintain and improve soil fertility.
- Perform soil analysis and balance nutrient levels, using fertilizers (whether organic or inorganic). Read more about soil test.
- Maintain and build soil organic matter: Apply compost and plant cover crops.
- Manage soil salinity, avoid accumulation of salts.
- If soil pH is too low – use lime to increase soil pH (remember that increasing soil pH may take years). Read more about soil pH and acidity.
- Apply proper irrigation practices to avoid leaching of essential nutrients and runoff.
- Consider tillage to break up large soil aggregates and compacted soils. Tillage also stimulates the decomposition of organic matter.